Generic RTL-SDRs that come with a plastic enclosure can be prone to picking up interference directly via the PCB itself. Higher end RTL-SDRs generally come with a metal enclosure.
Thanks to Alan R. for submitting a low cost idea he's come up with for shielding his plastic RTL-SDR dongles. Alan writes:
I’ve used this attached method with quite a bit of success for shielding the RTL dongle. It’s just a fizzy orange tube with two holes drilled at each end and some sticky metal insulating tape, which can be bought at any DIY store. Once the USB adapter and RTL dongle are inside they fit snuggly and any standard printer cable with reasonable length works well. Usually this allows for shorter coax connections which again helps keep the signal to noise level reasonable. I opted for a USB with a ferrite core at either end and I also added one to the coax - just because.
If you leave the antenna detached and tune in to any FM radio station you get a strong signal, and as soon as you put the RTL-SDR inside the insulated tube the signal stops. Needless to say if you plug the antenna in then the FM radio comes through with a strong signal. It certainly helps cut down a lot of FM broadcast noise (cheaply!)
I’m not using any band stop / band pass / pre amps and currently I’m receiving loud and clear satellite transmissions - NOAA / Meteor 2. I can even grab the telemetry from AO-73 Funcube with no problems. As the USB is shielded too and is away from the computer it also helps. Plugging the dongle directly into the computer tends to pick up a lot of unwanted noise.
The only thing to watch is it can get a little hot, so some common sense when using it (especially on a hot day). The other advantage is the weather proofing should you get caught in rain!
The SDRplay RSP is a $149 USD software defined radio that many consider as a next step upgrade from the RTL-SDR. See our recent review for a comparison between the Airspy, SDRplay RSP and HackRF.
One problem with the SDRplay RSP is that it comes in an unshielded plastic enclosure. This means that strong interfering signals can pass through the enclosure and cause interference, making any filtering done on the antenna less effective. Recently Tom Naumovski wrote in to us to let us know that he has been experimenting with a simple fix that involves shielding his RSP with adhesive copper tape. (Tom carefully notes that doing this may void the warranty). Tom simply wraps the plastic enclosure with conductive copper tape, making sure that electrical contact is made between the copper shielding and RSP ground (e.g. making sure the RSP USB and SMA ports make electrical contact with the copper tape)
After shielding the RSP, Tom tested the shielding effectiveness by using his shielded RSP with no antenna connected to try and pick up an interfering tone transmitted by his HackRF SDR. He collected the results in a pdf file. The results clearly show that the shielded RSP does not pick up, or significantly reduces the power of the HackRF’s interfering tone.
He also removed the USB plugs on the RTL-SDR’s and wired them into a USB B plug mounted to the case, making sure to wind the USB power cables through several turns of ferrite core in order to reduce USB noise. Finally he also added a power switch to the USB connections, to be able to easily power off the units when not in use.
Over on YouTube user Cameron Conover has uploaded a video where he tests the effectiveness of the optional HackRF RF shield and also shows how to install it. The shield solders on to the RF front end of the HackRF and is intended to block signals from entering the device other than through the antenna.
To test the effectiveness of the shield Cameron uses a signal generator to transmit a test signal at 406 MHz. He shows that without the shield in place the HackRF with a 50 Ohm terminator on the antenna input strongly picks up the test signal. After soldering in the shield and testing again, the test signal can no longer be picked up.
The programmer of Linrad, Leif (sm5bsz) has recently been experimenting with some modifications to the Airspy metal case. He discovered that the USB and RF input connections on the Airspy were not making good electrical connections to the metal case because of the paint on the case. These bad connections caused interfering broadcast FM to be received by the Airspy through the USB cable even when the antenna input was terminated with a dummy load. By sanding down the paint on the metal box to improve the connection he was able to significantly reduce the interference. He writes:
It is a good idea to make sure that both the SMA connector and the USB screen have a good electrical contact with the box. Grounding only the USB screen causes a severe degradation of the NF.
The findings here may also be useful for improving shielded RTL-SDR dongles.
In his latest tests he tried a metal outlet box as the case and saw improved results over the aluminium case. His conclusions seem to indicate that the aluminium box is not a good EMI shield. We’re not sure why he found these results, but one theory might be that because the aluminium case is anodized, it has a non-conductive surface, which might cause poor grounding.
Over on the Reddit thread discussing his work, there are some concerns about excessive crystal drift due to there being no ventilation holes. However, it seems that the general consensus is that lack of ventilation will not significantly affect crystal drift and may actually help to stabilise the crystal over time by keeping the internal temperature more constant.